1. Field of the Invention
The present invention relates to a sheet size detecting apparatus or an automatic original feeding apparatus adapted for use in an image forming apparatus such as a copying machine or a laser beam printer. More particularly it relates to an automatic original feeding apparatus provided with original size detecting means for detecting the original size by detecting the leading and trailing ends of the original in the course of conveyance, and also to an image forming apparatus equipped with such automatic original feeding device.
2. Related Background Art
In the conventional automatic original feeding apparatus for automatically feeding originals to a reading unit of an image forming apparatus such as a copying apparatus, the original supplied from an original stacker is transported to and fixed at the reading position, by a conveyor belt capable of forward and reverse rotation, and, after the image reading by a reciprocating motion of a light source (reading means), it is discharged by the conveyor belt onto the original stacker. In this operation, the discharged original is transported to the original stacker from a side same as that of entry of the original to the reading position. The above-mentioned original fixing method with switch-back motion of the original requires a time for interchanging the originals after image reading, and a faster transporting speed of the originals, if employed for increasing the image reading speed, will result in drawbacks such as noise and mechanical shocks.
In contrast to the original reading by the original fixing method explained above, there is proposed a flow reading method in which the reading means is fixed and the original is read in the course of flowing motion thereof. Because of the configuration of the original stacker, this flow reading method is applied for reading small-sized originals such as B5 or A4 size, and the originals on the original stacker are supplied to the original flow position from a side opposite to the side of feeding in the above-mentioned switch-back feeding. The supplied original is discharged to the original stacker through a closed path (CP) constituting a closed loop, and the original is read by the fixed flow means in the course of conveyance through said path.
In such closed-path feeding, the stack of the originals on the original stacker is pushed to the other end portion (closed path feeding portion) of said stacker by a shutter (moving means). Also on the closed-path feeding portion of the original stacker there is provided an original rear end sensor for detecting a large-sized original when it is placed on the stacker, and the original is fed from the side of the switch-back path when said sensor is activated.
The size of the original on the original stacker is detected, by bringing a pair of lateral plates for defining the lateral position of the originals into contact with the lateral edges of the original, by size sensors linked with said lateral plates.
In case of closed-path feeding of the original, it is conveyed with one of at least two conveying speed, i.e. a flow reading mode speed V1 and a fixed reading mode speed V2. In the closed convey path, there is provided a detecting switch for generating signals indicating the starting and ending times of the detection of the original size detection means. Said detecting switch is provided with an arm which crosses the original convey path and is rotated by the leading and trailing ends of the conveyed original to activate the detecting switch, and the detection time of the original size detecting means is controlled by the on/off operation of said switch to detect the original size.
In the conventional configuration explained above, however, the original size detection means for detecting the size of the original passing through the closed convey path has a mechanical lever to be rotated by the leading and trailing ends of said original as the actuator for the detecting switch, so that, although the switch turn-off time is constant by the returning rotation of said lever in response to the passing of the trailing end of the original, the moving amount of said trailing end after passing the original varies depending on the conveying speed of the original.